The present invention relates generally to gas chromatography and particularly to controlling the operating pressure of the pneumatic portion of a gas chromatography apparatus. Gas chromatography (GC) entails the analytical separation of a vaporized or gas-phase sample. In a GC system, a chemically inert carrier gas such as hydrogen, helium or nitrogen is utilized as the mobile phase for elution of the analyte sample in an analytical (or sample) column. The carrier gas is introduced into the system upstream of the analytical column. The sample is typically injected into the gas stream at or near the head of the analytical column and is thus carried by the carrier gas through the analytical column. The analytical column is typically housed in a thermally controlled oven or alternatively may be directly heated such as by controlled electrically resistive heating means. The analytical column may be constructed of stainless steel, glass, fused silica, Teflon®, or the like. The analytical column may be of the packed or open tubular (capillary) type. The analytical column contains a stationary phase (particles, films or layers of a selected composition) by which different components of the sample are retained differently. Thus, as the sample flows through the analytical column it becomes separated into discrete components of differing analytical (qualitative and/or quantitative) significance. The eluent from the analytical column flows to a gas detector provided with the GC system. Various types of detectors may be employed such as, for example, a flame ionization detector (FIO), thermal conductivity detector (TCD), etc. The choice of detector often depends on the sample being analyzed. Moreover, the type of carrier gas utilized often depends on the type of detector utilized. Additionally, depending on detector design, the carrier gas may be utilized as a reference gas by also flowing the carrier gas through a separate reference column (not containing the sample) to the gas detector under the same conditions (e.g., temperature, pressure, stationary phase, etc.) as the analytical column. Generally, the gas detector is of a type responsive to a property of the separated analytes (e.g., concentration) and converts the outputted flow of separated analytes to electrical measurement signals, which are then transmitted to a data processor. The data processor derives peak information or other useful analytical information from the measurement signals received. When a reference column is utilized, measurements of the eluent of the reference column are also taken into account in the data acquisition.
A GC system typically utilizes one or more gas flow (flow rate and/or pressure) controllers to control (switch on and off) the flow of carrier gas to the GC column. For the GC system to operate properly, the carrier gas must flow through a GC column at a particular working pressure (i.e., column head pressure). The gas flow controller(s) may be of either the mechanical or electronic type. Conventionally, both types of gas flow controllers regulate pressure relative to ambient pressure, thus making them susceptible to short-term ambient pressure fluctuations caused by natural conditions (wind, drafts) or other conditions (e.g., opening/closing doors in a laboratory, etc.). From the perspective of data acquisition, these pressure fluctuations cause disturbed baselines and high noise levels, which in turn result in higher detection limits and reduced repeatability of results. These problems have been particularly noted in cases where a GC system utilizes a TCD as the detector. A TCD typically includes ambient pressure outlets, thereby making the detector also susceptible to ambient pressure fluctuations. In addition, capillary GC systems in which the carrier gas flows are very low are particularly sensitive to ambient pressure fluctuations.
FIG. 1 is a schematic view of an example of a conventional GC system 100. A carrier gas supply 104 is operated to establish a flow of carrier gas to a gas flow controller 108 (typically a pressure controller). In this example, the original carrier gas flow is split into two carrier gas flows, either by the gas flow controller 108 or by a separate flow splitter (not shown). One of the carrier gas flows is inputted to an analytical column 112 while the other carrier gas flow is inputted to a reference column 116. The columns 112, 116 are enclosed in a suitable housing (not shown), which may include an oven as noted above. A sample injector 120 positioned at or near the head of the analytical column 112 introduces a gas-phase or vaporized sample into the carrier gas flow, whereby a sample-bearing (sample and carrier) gas is flowed through the analytical column 112 while a carrier-only gas is flowed through the reference column 116. The respective distal ends of the analytical column 112 and the reference column 116 are fluidly connected to a TCD 124. The TCD 124 receives the analytically separated sample gas effluent and the reference gas effluent, generates electrical signals based on measurements of thermal conductivity, and transmits the signals to a data acquisition system 128 for further processing, readout/display, etc. By way of background, the TCD 124 typically includes a four-element Wheatstone bridge in which the sensing elements are temperature-sensitive (e.g., resistive filaments, semiconducting thermistors, etc.). Some of the sensing elements are exposed to the sample gas flow while the other sensing elements are exposed to the reference gas flow. The resistances of the sensing elements vary in response to temperature changes. The temperature of each sensing element depends on the thermal conductivity of the sample gas or reference gas flowing around the sensing element. Thermal conductivity in turn depends on gas composition and thus may be correlated to the concentration of a particular gas molecule. The TCD 124 in this case is arranged such that the thermal conductivity of the reference gas (i.e., the carrier gas component of the sample gas as well as the carrier gas-only reference gas) is canceled out from the measurement signal outputted to the data acquisition system 128. Among other tasks, the data acquisition system 128 correlates the measurement signals to the concentration of the sample gas as it flows in analytically separated form from the analytical column 112, thereby enabling the generation of information-rich sample peaks the data output.
Of particular note for purposes of the present subject matter, the pressure controller 108 includes an ambient pressure reference channel 132 and the TCD 124 includes a sample gas outlet vent 134 and a reference gas outlet vent 136. These features allow gases to vent to atmosphere, whereby the entire pneumatic system of the GC system 100 essentially operates as an open system that is exposed to the ambient conditions in which the GC system 100 resides. It thus can be seen that the conventional GC system 100 is highly susceptible to ambient conditions, particularly the pressure fluctuations that may occur as described above. Prior solutions to minimizing the influence of ambient pressure fluctuations have been mainly focused on reducing ambient pressure fluctuations in the direct environment in which the GC system 100 operates (e.g., reducing air currents in the laboratory) and not on making improvements to the GC system 100 itself.
In view of the foregoing, there is a need for providing GC apparatus and methods that render the deleterious effects of ambient pressure fluctuations negligible.